Koji, Kiwamu with help from Cheryl and JoeD
As a part of the high power preparation, we swapped the beam splitter, IO_MB_M2, from the high transmissive one (T10%) to the low transmissive one (T2%). As expected, this replacement had caused misalignment in both the main and ALS pick off paths. We realigned them. Now the IMC is back to operation and is locking. The ALS beam also successfully comes out to ISCT1.
Preparation of the mount and BS:
We started today from peeling the first contacts off from the BSs. We had two identical BSs, one of which was supposed to be a spare. We removed the first contacts from both BSs and found that one showed many residues on the HR surface. Therefore we decided to use the other one which had much less residues and asked Joe to clean a couple of tiny residue off of it. Also, we asked Joe to apply new first contacts on the one which had many residues so that someone can use it for useful purpose in some future.
Cheryl already had Tyler to modify a PSL-custom-made base mount to nicely accommodate a Ultima mirror holder. So we took it from her desk and mount a left-handed 1 inch Ultima equipped with three actuation knobs so that we can control a translation in addition to the usual two angular degrees of freedom. Note that the old BS which had been in place is a 2 inch BS and that's why we needed to swap not only optic itself but also its mount and base.
Alignment of the main path:
The alignment of the main path went very smooth.
Before doing anything, we placed an iris on IOT2L as a reference. It was put before the length diode. After installing the new BS, we realigned the main path using Cheryl's irises on the PSL table. The important and excellent point is that we touched only this particular BS and didn't touch anything else for this alignment. After centering the beam on the irises on the PSL table, we then took a look at the beam on the table. The beam was already landing on the table and in fact it hit the length diode. So we coarsely centered the beam on the diode by steering the BS. At this point, the alignment was good enough for us to lock IMC. A fine alignment was then done by maximizing the IMC power build up. We could get as high power as it used to be in the transmitted light of IMC and hence we were satisfied with this alignment. After turning the ASC loops on, we found that WFSs' DC centering were excellent. I would say this alignment recovery was very successful.
Alignment of the ALS path:
The alignment of the ALS path was not as easy as the main path.
Because the thickness of the BS changed, the beam spot on the first mirror in the ALS path was translated. The translation was so big that the beam was not making through the faraday any more. We tried to recover the alignment without shifting the position of the Faraday and by touching the first steering mirror, but it turned out that it was quite difficult to do it. So eventually, we decided to shift the Faraday. The first steering mirror was steered so that the beam hits the center of the next steering mirror. Then we moved the Faraday, which is between the first and second mirrors, so that the beam goes through it. The transmissivity of the Faraday was measured to be approximately 94% (= 24.50 mW / 26.03 mW) which is acceptable. The we tweaked the second and third mirrors to steer the beam on an iris on ISCT1. After the tweak was done, we assessed the risk of clipping by introducing a large angle misalignment in the third steering mirror and confirmed that the beam is in the middle of the clipping aperture.
